Acetic acid is a well-known commodity chemical which has many industrial uses.
Various methods for producing acetic acid industrially are well known, many such processes produce propionic acid, either as a by-product or a co-product. In order for the acetic acid to meet necessary specifications for use and/or sale, it is often necessary to subject the acetic acid product to a process for the removal of propionic acid.
EP 0 849 250 A1 discloses a process for the production of an acetic acid process stream comprising less than 400 ppm propionic acid and less than 1500 ppm water, which process comprises the steps:—
(a) feeding methanol and/or a reactive derivative thereof and carbon monoxide to a carbonylation reactor in which there is maintained during the course of the process a liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide cocatalyst, optionally one or more promoters selected from the group consisting of ruthenium, osmium, rhenium, cadmium, mercury, zinc, gallium, indium and tungsten, a finite amount of water at a concentration of less than about 8% by weight, methyl acetate, acetic acid, and propionic acid by-product and its precursors;(b) withdrawing liquid reaction composition from the carbonylation reactor and introducing at least part of the withdrawn liquid reaction composition, with or without the addition of heat, to a flash zone to form a vapour fraction comprising water, acetic acid product, propionic acid by-product, methyl acetate, methyl iodide and propionic acid precursors, and a liquid fraction comprising involatile iridium catalyst, involatile optional promoter or promoters, acetic acid and water;(c) recycling the liquid fraction from the flash zone to the carbonylation reactor;(d) introducing the vapour fraction from the flash zone into a first distillation zone;(e) removing from the first distillation zone at a point above the introduction point of the flash zone vapour fraction a light ends recycle stream comprising water, methyl acetate, methyl iodide, acetic acid and propionic acid precursors which stream is recycled in whole or in part to the carbonylation reactor, and(f) removing from the first distillation zone at a point below the introduction point of the flash zone vapour fraction, a process stream comprising acetic acid product, propionic acid by-product, and less than 1500 ppm water and,(g) if the process stream removed in step (f) comprises greater than 400 ppm propionic acid introducing said stream into a second distillation column, removing from a point below the introduction point of the stream from (f) propionic acid by-product and from a point above the introduction point of the stream from (f) an acetic acid process stream containing less than 400 ppm propionic acid and less than 1500 ppm water.
Only limited details of the configuration and operation of the optional second distillation column in step (g) of EP 0 849 250 A1 are provided.
WO 2009/042078 A1 discloses a method and apparatus for making acetic acid with improved purification. The improvements in the purification provided in WO 2009/042078 A1 are not directly related to the configuration and operation of the heavy ends column disclosed therein, and only limited details of the configuration and operation of the heavy ends column are provided.
WO 2012/014393 A1 provides a process for producing acetic acid with a high purity. Disclosed in WO 2012/014393 A1 is a second distillation column which separates at least part of a higher boiling component (e.g., propionic acid) from the bottom of the column. Only limited details of the configuration and operation of the second distillation column of WO 2012/014393 A1 are provided.
US 2010/0063319 A1 provides a process for producing acetic acid, the process comprising the carbonylation of methanol to form a reaction mixture comprising a catalyst, catalyst stabiliser, acetic acid, methanol, methyl iodide, methyl acetate, water and carbon monoxide, and introducing at least a portion of the reaction mixture to a distillation column to separate into a bottom stream comprising the catalyst and catalyst stabiliser, a sidedraw stream comprising acetic acid and water, and an overhead stream comprising methanol, methyl acetate, methyl iodide and water. The process provided by US 2010/0063319 A1 eliminates the use of a flash tank. The sidedraw stream removed from the distillation column is optionally subjected to further purification such as drying-distillation to remove water and heavy-ends distillation to remove heavy impurities such as propionic acid. US 2010/0063319 A1 does not provide any details regarding the configuration or the operation of the optional heavy-ends distillation.
The operation of a heavy ends column for the removal of propionic acid from an acetic acid product requires large amounts of energy due to the fact that the by-product propionic acid has a higher boiling point than the acetic acid.